1. Field of the Invention
The invention relates to a method for fabricating a flat panel display that comprises thin film transistors with different carrier mobility.
2. Description of the Prior Art
An OLED uses organic luminous devices, such as organic light-emitting diodes, as the light source of the display. An organic luminous device is an electrically driven lighting element having a brightness that depends on the magnitude of a related current. The magnitude of the brightness, also called the gray-scale value, is controlled by the magnitude of the driving current of each sub-pixel, and the sub-pixels are arranged in a matrix or array in an OLED, which is called a matrix display. As a result, the OLED utilizes this characteristic of the organic luminous devices to generate red, blue, and green lights with different intensities of gray level to produce stunning images.
The matrix display is classified as a passive matrix or an active matrix display according to the driving method. Passive matrix displays adopt the method of driving the scan lines of the display in sequence, driving pixels in different rows sequentially. Since the light-emitting time of each pixel is restricted by the scanning frequency and the numbers of scan lines, the passive matrix method is not suitable for large-sized and high dots-per-inch (dpi) displays with a high amount of scan lines. In contrast, active matrix displays possess an independent sub-pixel circuit for each sub-pixel, which includes a capacitor (Cs), an organic light-emitting element, and at least two TFTs that are used to adjust the OLED driving current. With this arrangement, even in large-sized and high dpi displays, a steady driving current is provided for each sub-pixel.
FIG. 1 is a schematic diagram of an active matrix OLED panel 10 according to the prior art. As shown in FIG. 1, a panel display 12 comprises a matrix composed of a plurality of data lines 22 (such as D1, D2, and D3) and scan lines 24 (such as S1, S2, and S3). The panel display 12 also comprises a plurality of sub-pixel circuits 26, wherein each sub-pixel circuit 26 has at least two TFTs, a storage capacitor (Cs), and an organic light-emitting element 20 at each intersection of a data line 22 and a scan line 24. Each sub-pixel circuit 26 is electrically connected to a corresponding data line 22 and a corresponding scan line 24 for driving the organic light-emitting element 20 in the corresponding sub-pixel. The data lines D1, D2, and D3 connect to a data line driver 16 for receiving an image data signal, and the scan lines S1, S2, and S3 connect to a scan line driver 18 for receiving a switch/address signal.
FIG. 2 is a schematic diagram of the sub-pixel circuit 26 shown in FIG. 1. As shown in FIG. 2, the sub-pixel circuit 26 comprises a switching TFT 28, a driving TFT 30, and a storage capacitor 32. In the prior art, generally, the switching TFT 28 and driving TFT 30 are NMOS and PMOS transistors respectively. The gate of the switching TFT 28 is electrically connected to the scan line 24, and the source, point A, of the switching TFT 28 is electrically connected to the data line 22. In addition, the gate, point B, of the driving TFT 30 is electrically connected to the source of the switching TFT 28 and one end of the storage capacitor 32. The source, point C, and the drain, point D, of the driving TFT 30 are electrically connected to the organic light-emitting element 20 and an external power supply respectively.
The driving method of the conventional OLED panel 10 is described in the following. Referring to FIG. 1 and FIG. 2, when a video data signal is inputted into a control circuit 14, the control circuit 14 generates corresponding control signals to the data line driver 16 and the scan line driver 18 according to the video data of each sub-pixel. Then, the scan line driver 18 outputs corresponding scan signals to each scan line 24 (S1, S2, . . . and Sn) in sequence for turning on the sub-pixel circuits 26 in each row in order and thereby making the corresponding pixels perform the display operation. For example, when the OLED panel 10 is going to drive a sub-pixel positioned in the intersection of D2 and S2, the control circuit 14 sends a scan signal through the scan line 22 to the gate of the switching TFT 28 of the sub-pixel circuit 26, and sends a corresponding data signal, normally a voltage signal with a predetermined intensity, to the drain of the switching TFT 28 through the data line driver 16 and the data line 22 according to the video data.
Since the switching TFT 28 conduct, the video data will charge the storage capacitor 32 to have a first voltage through the switching TFT 28 and generate a corresponding driving current at point C, which is then output to the organic light-emitting element 20 to make the light-emitting element 20 generate light beams with a corresponding brightness. When the OLED panel 10 performs in continuous operation, such as driving the sub-pixels in the next row, the storage capacitor 32 still has the first voltage although the voltage on scan line S3 decreases resulted in the switching TFT 28 becoming closed. Therefore, the driving TFT 30 still conducts. Furthermore, since there is a voltage difference between point D and point B, a current is continuously passing through the driving TFT 30 to the organic light-emitting element 20 to continuously keep the organic light-emitting element 20 emitting light beams.
In conclude, although a sub-pixel circuit may have various design structures of an AMOLED panel and the amount of TFTs in a sub-pixel circuit may be different, a sub-pixel circuit usually contains at least two TFTs for driving the organic light-emitting element, such as the driving TFT 30, and for switching the sub-pixel, such as the switching TFT 30. In the driving method of an OLED panel as described above, the sub-pixel circuit used for driving the organic light-emitting element is one of the key devices for displaying video data on time and correctly. Furthermore, since the driving TFTs and the switching TFTs control the switch of each sub-pixel and the organic light-emitting element of each sub-pixel, the quality of the switching TFTs and driving TFTs is a key factor in the performance of the OLED panel.
Generally, the switching TFTs and the driving TFTs are low temperature polysilicon (LTPS) TFTs and fabricated simultaneously with the same fabrication processes in the prior art. The polysilicon layer of the channel region of one LTPS TFT is formed under a low temperature. The prior-art method for forming the channel region includes using laser beams with various energies or utilizing a laser machine including a mask to mask and adjust the laser beams so as to form the polysilicon layer.
Since the switching TFTs and the driving TFTs have different functionalities in a sub-pixel, they actually require different electrical properties in operation. For example, one switching TFT is used for turning on its corresponding sub-pixel, and therefore it requires a high carrier mobility and a high driving current. On the other hand, a driving TFT is used to drive the organic light-emitting element and controls the brightness of the light beams of the organic light-emitting element in the sub-pixel. Accordingly, all the driving TFTs in the flat panel display should have similar driving capability, and the carrier mobility of the driving TFTs should be moderate for maintain the lifetime of the organic light-emitting elements longer. However, the channel regions of the driving TFTs and the switching TFTs are conventionally formed by a laser irradiation in the fabrication method of the prior art, thus the grain structures of the channel regions have a large carrier mobility, about 100 cm2/V·s and a large deviation. Therefore, the driving TFTs including the grain structure of the channel regions with a large deviation may have different carrier mobility in a large range, which affect the organic light-emitting elements in all of the sub-pixels may have different brightness of largest magnitude. Since the driving TFTs have different driving currents in a large range, it is difficult for the whole flat panel display to have a brightness uniformity, which is called a mura problem. Furthermore, the lifetime of the organic light-emitting elements in each sub-pixel is hard to be controlled. Accordingly, the witching properties of the switching TFTs and the driving TFTs cannot be satisfied simultaneously.